Lower limb loss causes severe mobility deficits that affect many other aspects of lives of amputees, including decreased community involvement and depression. One major cause for these mobility challenges is the lack of small, lightweight leg prostheses that can provide power like the human neuromuscular system. A person's walking or other ambulation gait can be cyclical, with a stance phase and a swing phase. Stance phase is the part of the gait cycle when weight is borne by the leg. Swing phase is the part of the gait cycle when the foot is in the air and weight is borne by the opposite leg.
The human ankle is important for walking because it provides over half of the energy required to move the body forward. The ankle produces energy during part of the gait cycle known as “push off” or “powered plantarflexion.” During push off, the calf muscle contracts and propels the body into the next step. During the remainder of the gait cycle, the ankle produces little to no power.